Various types of positioning systems for determining a position based on radio signals are known in the art. For example, satellite navigation systems allow autonomous geospatial positioning with virtually global coverage. Global navigation satellite systems (GNSS) provide GNSS receivers with the capability to determine their location based on positioning signals transmitted from the GNSS satellites in terms of longitude, latitude and altitude, to within a few meters or even centimeters.
GNSS based positioning has a wide range of applications, including navigation and tracking and automatic positioning.
Generally, for determining its position, a GNSS receiver first determines distances to a plurality of GNSS satellites. Each individual distance measurement made by the receiver to a satellite located in a known orbit position traces the receiver on the surface of a spherical shell at the measured distance from the satellite. By taking several such measurements and determining an intersecting point of the spherical shells, a position fix can be generated. The distance measurements to the satellites are based on a time of flight measurement of positioning signals transmitted by the satellites to the receiver and thus the measurements depend on an exact timing. Normally, three distance measurements to three known satellite positions are sufficient to resolve a receiver position in space, however, with time being the fourth unknown in the equations, measurements on four satellites are needed to determine the position of the receiver.
The orbit position of the satellite may be determined based on a data message superimposed on a code that serves as a timing reference. The receiver can compare the time of broadcast at the satellite encoded in the transmission with the time of reception measured by an internal clock at the receiver, thereby measuring the time of flight to the satellite. Some GNSS systems provide satellites that transmit a code with a timing reference, enabling a receiver to compare a successively delayed internal replica of this code with the received code from the satellite, and, when determining a match of the codes, to determine the amount of delay. This type of code based positioning allows accuracies within several meters.
For higher accuracies Real-Time Kinematic (RTK) positioning is known. RTK positioning employs measurements on a carrier phase of the positioning signals from the satellites. In RTK it is not a code that is compared with a delayed internal version of the code, but the carrier itself is used in the comparison process. By using the phase of the carrier signal from the satellite centimeter accuracy positioning can be achieved.
Various error sources, however, affect the absolute positioning accuracy. As noted above, the exact time of flight of the signal from the satellite to the receiver station must be measured, which may be in the range of e.g. 0.06 seconds from a satellite directly above a receiver. In order to make the time measurements as accurate as possible, GNSS satellites generally include several atomic clocks providing a highly accurate time reference. However, still, even atomic clocks suffer from a certain time error that constitutes an error source in the measurements that has to be observed when desiring centimeter level accuracy. Other error sources deteriorate the positioning result, including propagations delays introduced by the troposphere and ionosphere, orbit errors in the satellite positions, relativistic effects, as known in the art.
To improve the accuracy of the estimation, systems for example performing a positioning based on carrier phase measurements often provide reference data from another source to a receiver or rover station, e.g. via ground based radio transmission, in order to enable the receiver or rover station to eliminate the positioning errors introduced by the error sources. For example, a reference station with its exact position known may be used to eliminate errors in the measurements taken by a rover station, if in the approximate same geographical region. Measurements made at the reference station can then be transmitted from the reference station to the rover station and used thereat to eliminate the errors in the position determining process. For example, if it is assumed that the errors in the receiver measurements and reference station measurements are the same, the rover station may eliminate the errors by determining a difference between the measurement at the receiver and at the reference station.
While having the advantage of improved position determination results if the rover station uses reference data from reference stations and thus facilitating applications requiring highest accuracy, the positioning accuracy now relies on the permanent availability of the reference data. Especially on remote construction sites or in agricultural applications it may, however, be difficult to provide the reference data at all times to a rover station, causing a deterioration of the position determination during the times of unavailability of reference data which obviously is undesirable or even dangerous, for example in automatic positioning applications.